Summary of Work: Eukaryotic genomes contain not only stable genes, but also genetic elements that can be mobilized through a process of transposition. These transposable elements are often maintained in high copy number and their movement has been postulated to play a role in chromosome instability and mutagenesis. In addition, transposition of the retroposon, HeT-A, in Drosophila is necessary for maintenance of chromosome length. We are developing two experimental model systems that use phenotypic identification to quantitate transpositional events with the aim of determining what genetic factors, chemicals, or other environmental insults lead to retroposon transposition. In one system, using the retroposon HeT-A in Drosophila, the w gene containing an inverted intron is used as a genetic marker. Transposition will be monitored in vivo. This system is partially developed and will be used to design a similar system in the mouse. In the mouse, GFP will be used as the genetic marker, and transposition of an L1 element will be monitored in vitro. The project is supported by an IRA and began in February. The marked transposable elements are currently being constructed.